The invention relates to precision surface measurement, including techniques to measure surface characteristics of precision optical components unbiased by a measurement system.
The need to determine the errors in a part (e.g., an optical component, such as a lens or mirror), unbiased by errors in a measurement system, is a long-standing problem. One general approach is to measure multiple objects in different combinations and then to determine the contributions from each object individually, such as in the 3-Flat test. The 3-Flat test has a variety of limitations; one of the most significant is the need for three, nominally identical parts to test.
The second general approach is to perform a measurement of the surface of a “test part” in a measurement system, and then to displace or “shear” the test part relative to the measurement system. Differencing the two measurements cancels the contribution (bias) of the measurement system and leaves one with an approximation of the derivative of the surface in the same direction as the shear motion. An estimate of the surface of the part under test unbiased by the measurement system error may be derived from the difference data.
Within the family of shearing methods, there are two general approaches that have been used: lateral shear and rotational shear. Lateral shear in two orthogonal directions results in an estimate of the gradient of the part under test; however, the use of lateral shear alone can be sensitive to drift resulting in errors proportional to the array size in the estimate of the test part. Rotational shearing methods have been widely reported and have been shown to be robust for the determination of the rotationally varying surface errors; however, rotational shearing methods alone do not typically allow one to determine the rotationally invariant errors (e.g., mean radial profile) of the test part and instrument separately from one another.